Microwave assisted chemistry refers to the use of microwaves to initiate or accelerate chemical reactions. Microwave assisted chemistry is particularly useful in heating materials that are responsive to microwave radiation because under most circumstances, the resulting heating takes place much more rapidly than it would if the reactions were initiated or accelerated using more conventional heating techniques such as convection or conduction heating.
Microwave assisted chemistry can be used in a variety of chemical processes including moisture determination, ashing, digestion, extraction, and others. Under some circumstances, these various techniques are preferably or necessarily carried out in sealed vessels which, because of the generation or expansion of gases inside, must be able to withstand high pressures.
Accordingly, a number of pressure vessels have been developed that are suitable for high-pressure microwave assisted chemistry. Such vessels are typically formed of microwave transparent materials that offer the structural capabilities required to withstand such high pressures. High-strength polymers are exemplary of such materials and offer the required microwave transparency and resistance to chemical attack. Such materials tend to be brittle, however, so that failure under pressure tends to destroy the vessel quickly and release its contents suddenly.
One recent advance in the construction of such vessels has been to use a composite sleeve as one of the outer portions of the reaction vessel. The composite is formed of several alternating layers of plastic (polymer) and fabric. In such a composite structure, the materials synergistically complement each other by providing characteristics unavailable from the other material, and by providing a structure with characteristics better than either material alone In the case of sleeves for microwave vessels, the plastic portions of such a vessel offer chemical resistance and structural strength. The fabric portions offer additional strength as well as flexibility and the ability to change shape without breaking or shattering. Accordingly, when plastic-fabric composite vessels fail under pressure, they tend to fail rather gently. Stated differently, a fabric vessel, even if it could be constructed to hold gases, would never offer the strength required for high-pressure conditions. Alternatively, engineering resins and other materials can withstand high pressures, but tend to fail by shattering. When used together in a composite structure, however, the combination provides the strength for maintaining a high pressure in the vessel, while preventing shattering should the plastic fail.
Versions of such composite fabric vessels are disclosed, for example, in U.S. Pat. Nos. 5,427,741 and 5,520,886, both of which are commonly assigned with the present invention. Another version is set forth in co-pending and commonly assigned application Ser. No. 09/062,858, filed Apr. 20, 1998, the contents of which are incorporated entirely herein by reference (“the '858 application”).
As composite pressure vessels have become more widely used because of their advantages, certain characteristics have become more evident that can be improved upon. In particular, and taking for example the vessel structure illustrated in the co-pending '858 application, the flexible nature of the woven fabric layers tends to be such that if the vessel is exposed to high pressure, it may distort slightly. The vessel's characteristics are such that it will stay distorted even after the pressure is removed or released. By “distorted,” it will be understood that only a very slight change of shape may have taken place, sometimes as little as 0.001 inch. Nevertheless, when dealing with gases, such a change in dimension is enough to prevent the vessel from maintaining an effective seal under high pressure.
Additionally, in the vessel illustrated in the '858 application, the lid for the reaction portion of the vessel is sealed to the top of the vessel using a flat surface-flat surface contact arrangement (e.g., FIGS. 2 and 4 thereof). As in the case of slight flexing of the composite sleeve, slight deviations from the flat-on-flat contact can allow gases to escape. In some cases such self-venting is desirable and helps keep a reaction at or within desired pressure limits. In other cases, however, unintended venting can release gases (including reagents) and prevent the intended reaction from taking place.
Accordingly, a need exists for pressure vessels that incorporate the advantages of protective composite sleeves, but that improve upon the characteristics of the present vessels and reduce the possibility for distortion or leakage.